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If 5,500 trees fall in Pasadena: The 'Tree Failure' windstorm report

A boy whose school was closed climbs fallen trees on Green Street after strong Santa Ana Winds that are cauing the worst local wind damage in decades on Dec. 1, 2011 in Pasadena. David McNew/Getty Images

Prepared for the city's Department of Public Works, the study examines data on nearly 5,500 trees that were damaged in the historic windstorms -- with wind speeds reaching 100 miles per hour within the City of Pasadena -- on November 30 and December 1, 2011.

An analysis of tree failures by the Davey Resource Group showed the city’s urban forest suffered "severe losses" with approximately 9% of the trees damaged in the high winds. Costs to the city are projected to be in the tens of millions of dollars with hundreds of light poles and buildings also damaged in the storm.

The City of Pasadena maintains that it maintains about 57,500 trees in over 64,500 city locations. The report asserts that the "unpreventable wide variety of damage to the urban forest structure" was primarily caused by wind intensity and direction, and that the existing tree maintainence program prevented more damage from occuring.

Some report findings:

56% of the damage to the urban forest was branch failure.

30% of the damage to the urban forest was whole tree failure.

Top six species to suffer the most damage were live oak, Chinese elm, southern magnolia, camphor, Queensland pittosporum and holly oak

Queensland pittosporum was the poorest performing species with over 50% of its population suffering damage.

Most affected trees were in fair or better condition at the time of the storm.

Wind speed and direction was determined to be the major contributing factor to damage.

Current and past pruning practices did not contribute to the extent of damage

Broad canopy trees had more failure types than columnar trees.

Whole tree failures were dominated by soil failure, not root failure.

Planting space and/or the presence of irrigation were not contributing factors to tree failure.

The windstorm was severe and extremely rare for Pasadena.

For more on the the City of Pasadena's "Master Street Tree Plan," and all the data you can shake a stick at, download the full report here.